LIMNETIC COMMUNITY 125 



similar to that of the large lakes and rivers. Oxygen is usually abundant 

 in the surface waters, but is often wanting in the bottoms of lakes (74) 

 with incomplete summer circulation. Muck bottoms in deep water 

 or in bays have little or no dissolved oxygen. Dissolved nitrogen is 

 important, but has been little studied. In the open water light and 

 pressure are governed by the same factors as in the large lakes (see 

 pp. 62-64). The bottom in small lakes varies with exposure to waves. 

 Where the waves are eroding, the bottom is stony or sandy; where deposit- 

 ing, it contains silt and humus. There are often deposits of marl, which is 

 a calcium carbonate deposit, frequently reaching a depth of 18 feet in the 

 Indiana lakes. It frequently reaches to the surface of the water, but 

 when it does so is often covered by muck. Muck bottom is common in 

 the deeper water and in bays. The vegetation in such lakes is very much 

 like that in base-level streams. The vegetation of the shores of rivers 

 like Fox River is duplicated in these lakes, and in fact, small lakes are 

 strictly comparable to sluggish rivers in many respects. We have 

 patches of vegetation, patches of sand and gravel bottom, but also much 

 bottom which has more organic matter than river silt. The principal 

 difference is that currents in the lakes vary with the wind, and in sluggish 

 streams are mainly in one direction. 



II. COMMUNITIES OF SMALL LAKES 



(Stations 30, 300, 31; Table XXVI) 



These are divided into the limnetic formation, the formations of 

 sandy and stony shores, the formations of muck bottom in shallow 

 water, the formations of the vegetation, and the formations of deep 

 water (anaerobic). 



I. THE LIMNETIC FORMATION (104) 



(List II) 



The limnetic formation of the smaller lakes is very similar to that of 

 the larger lakes. It is made up of the same groups, but with the addition 

 of a few pelagic insects such as the phantom larva (Corethra sp.). The 

 species of crustaceans, rotifers, and protozoa are different. The char- 

 acters of the formation are similar to those of Lake Michigan (p. 75). 



2. SHALLOW WATER FORMATIONS 



a) Terrigenous bottom formation (105). Vegetation, sparse or absent 

 water 0-3 meters. Crawling over the sandy bottom are usually found 

 caddis-worms (Goera sp. or Molanna sp.) (Figs. 70, 71). These forms 



